The field of the invention is coherent imaging using vibratory energy, such as ultrasound, and, in particular, systems and methods for shear wave dispersion ultrasound vibrometry (“SDUV”).
There are a number of modes in which ultrasound can be used to produce images of objects. For example, an ultrasound transmitter may be placed on one side of the object and sound transmitted through the object to an ultrasound receiver placed on the other side of the object. With transmission mode methods, an image may be produced in which the brightness of each pixel is a function of the amplitude of the ultrasound that reaches the receiver (“attenuation” mode), or the brightness of each pixel is a function of the time required for the sound to reach the receiver (“time-of-flight” or “speed of sound” mode). In the alternative, the receiver may be positioned on the same side of the object as the transmitter and an image may be produced in which the brightness of each pixel is a function of the amplitude or time-of-flight of the ultrasound reflected from the object back to the receiver (“reflection,” “backscatter,” or “echo” mode).
There are a number of well known backscatter methods for acquiring ultrasound data. In the so-called “A-mode” method, an ultrasound pulse is directed into the object by an ultrasound transducer and the amplitude of the reflected sound is recorded over a period of time. The amplitude of the echo signal is proportional to the scattering strength of the reflectors in the object and the time delay is proportional to the range of the reflectors from the transducer. In the so-called “B-mode” method, the transducer transmits a series of ultrasonic pulses as it is scanned across the object along a single axis of motion. The resulting echo signals are recorded as with the A-mode method and their amplitude is used to modulate the brightness of pixels on a display. The location of the transducer and the time delay of the received echo signals locates the pixels to be illuminated. With the B-mode method, enough data are acquired from which a two-dimensional image of the reflectors can be reconstructed. Rather than physically moving the transducer over the subject to perform a scan it is more common to employ an array of transducer elements and electronically move an ultrasonic beam over a region in the subject.
The ultrasound transducer typically has a number of piezoelectric elements arranged in an array and driven with separate voltages. By controlling the time delay, or phase, and amplitude of the applied voltages, the ultrasonic waves produced by the piezoelectric elements (“transmission mode”) combine to produce a net ultrasonic wave focused at a selected point. By controlling the time delay and amplitude of the applied voltages, this focal point can be moved in a plane to scan the subject.
The same principles apply when the transducer is employed to receive the reflected sound (“receiver mode”). That is, the voltages produced at the transducer elements in the array are summed together such that the net signal is indicative of the sound reflected from a single focal point in the subject. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delays, or phase shifts, and gains to the echo signal received by each transducer array element.
There are a number of electronic methods for performing a scan using a transducer having an array of separately operable elements. These methods include linear array systems and phased array systems.
A linear array system includes a transducer having a large number of elements disposed in a line. A small group of elements are energized to produce an ultrasonic beam that travels away from the transducer, perpendicular to its surface. The group of energized elements is translated along the length of the transducer during the scan to produce a corresponding series of beams that produce echo signals from a two-dimensional region in the subject. To focus each beam that is produced, the pulsing of the inner elements in each energized group is delayed with respect to the pulsing of the outer elements. The time delays determine the depth of focus which can be changed during scanning. The same delay factors are applied when receiving the echo signals to provide dynamic focusing during the receive mode.
A phased array system commonly employs so-called phased array sector scanning (“PASS”). Such a scan is comprised of a series of measurements in which all of the elements of a transducer array are used to transmit a steered ultrasonic beam. The system then switches to receive mode after a short time interval, and the reflected ultrasonic wave is received by all of the transducer elements. Typically, the transmission and reception are steered in the same direction, θ, during each measurement to acquire data from a series of points along a scan line. The receiver is dynamically focused at a succession of ranges, R, along the scan line as the reflected ultrasonic waves are received. A series of measurements are made at successive steering angles, θ, to scan a pie-shaped sector of the subject. The time required to conduct the entire scan is a function of the time required to make each measurement and the number of measurements required to cover the entire region of interest at the desired resolution and signal-to-noise ratio. For example, a total of 128 scan lines may be acquired over a sector spanning 90 degrees, with each scan line being steered in increments of 0.70 degrees.
The same scanning methods may be used to acquire a three-dimensional image of the subject. The transducer in such case is a two-dimensional array of elements which steer a beam throughout a volume of interest or linearly scan a plurality of adjacent two-dimensional slices.
Characterization of tissue mechanical properties, particularly the elasticity or tactile hardness of tissue, has important medical applications because these properties are closely linked to tissue state with respect to pathology. For example, breast cancers are often first detected by the palpation of lesions with abnormal hardness. In another example, a measurement of liver stiffness can been used as a non-invasive alternative for liver fibrosis staging.
Recently, an ultrasound technique for measuring mechanical properties of tissues, such as stiffness and viscosity, called shear wave dispersion ultrasound vibrometry (“SDUV”) was developed. This SDUV technique is described, for example, in U.S. Pat. Nos. 7,785,259, and 7,753,847, which are herein incorporated by reference in their entirety. A focused ultrasound beam, operating within FDA safety limits, is applied to a subject to generate harmonic shear waves in a tissue of interest. The propagation speed of the induced shear wave is frequency dependent, or “dispersive,” and relates to the mechanical properties of the tissue of interest. Shear wave speeds at a number of frequencies are measured by pulse echo ultrasound and subsequently fit with a theoretical dispersion model to inversely solve for tissue elasticity and viscosity. These shear wave speeds are estimated from the phase of tissue vibration that is detected between two or more points with known distance along the shear wave propagation path.
One feature of the SDUV method is the use of a so-called “binary pushing pulse” that allows the operation of one single array ultrasound transducer for both motion excitation and the echo signal detection. For example, the transducer focuses ultrasound at one location, the “vibration origin,” to vibrate the tissue of interest and then electronically steers its focus to another location, a “motion detection point,” for echo signal vibration detection. Instead of continuously vibrating the tissue of interest, the “pushing” ultrasound is turned on during a vibration time period to vibrate the tissue and turned off to provide a time window for the pulse echo motion detection. When the pushing pulse is off, a series of short ultrasound pulses is transmitted to the motion detection locations and a corresponding series of echo signals is received and processed to determine the tissue vibration. This intermittent pulse sequencing strategy allows both the production of a shear wave and the monitoring of its propagation at the same time with a single array transducer.
A technical challenge for the SDUV method, however, is that the shear wave generated by the pushing ultrasound is small and difficult to detect with pulse echo ultrasound. For human applications, the intensity of an ultrasound push beam is limited by the FDA, such that the mechanical index (“MI”) should be lower than 1.9. In practice, MI is targeted to be at a lower value, such as around 1.4, to ensure that variations among different ultrasound scanners will never generate an MI that exceeds 1.9. This is important for practice because calibrating every ultrasound scanner to ensure MI is less than 1.9 is cost prohibitive, and requires a burdensome amount of time for a complete calibration. The result of using lower MI, however, is that vibratory tissue motion, which increases dramatically with MI, is too small, thereby making detection unreliable. The push beam duration can be extended to increase tissue motion; however, a push beam that is too long will interfere with subsequent detection pulses. By way of example, and referring to FIG. 1, long push pulses interfere with and corrupt two subsequent detection beams, leading to two missing data points in SDUV analysis. Missing data points that are recovered by interpolation introduce errors in the estimation of shear wave speeds at higher frequencies. The time interval between subsequent detection beams, termed the pulse repetition period, cannot be readily increased to avoid interference because the pulse repetition frequency, which is the inverse of pulse repetition period, for detect beams, PRFD, should be sufficiently high to detect shear waves at higher harmonics.
It would therefore be desirable to provide a method for shear wave dispersion ultrasound vibrometry (“SDUV”) that imparts vibratory motion to a tissue of interest with larger amplitudes than achievable with currently available techniques, but does so in a manner that does not significantly impede the ability to detect shear wave propagation and that remains within FDA safety limits for mechanical index.